1. Field of the Invention
The invention relates to a hybrid vehicle, and more particularly to control performed when an engine that is in a stopped state is started.
2. Description of Related Art
In a hybrid vehicle, the engine is intermittently operated according to running conditions of the vehicle, so that the engine is prevented from operating in a low efficiency region, for improvement of fuel economy. Accordingly, in the hybrid vehicle, the engine is automatically stopped and started during operation, resulting in increase of the frequency with which the engine is started. This makes it important to suppress vibration when the engine is started.
As a technology for suppressing vibration at the start of the engine, it is described in Japanese Patent Application Publication No. 2009-248619 (JP 2009-248619 A) to control cranking torque of a motor-generator that starts the engine by motoring, so that the engine speed promptly exceeds the resonance speed at which resonance occurs to the vehicle, when the engine is started.
In particular, in a hybrid vehicle having a power transmission mechanism including a transmission, the resonance speed changes according to the speed ratio of the transmission; therefore, control for increasing cranking torque is performed when the speed ratio is that of a high-speed gear position in which the resonance speed is increased, as described in JP 2009-248619 A.